The present invention relates to distance measuring devices and, more particularly, to distance measuring devices employing a modulated light source.
Light sources such as, for example, incandescent sources as disclosed in U.S. Pat. No. 3,778,157 and laser sources as disclosed in U.S. Pat. No. 3,792,928, have been used in distance measuring apparatus wherein the light is transmitted through optical fibers ending a distance away from a reflecting surface. Other optical fibers alongside the transmitting optical fibers receive the reflected radiation for transmission to a detector.
In U.S. Pat. No. 4,064,754, two optical paths are established through separate optical fibers in a device for measuring the depth of fluid in a tank. One of the fibers terminates in a reflective surface at a reference point in the tank. The other optical fiber transmits light into a measurement path which is variable in dependence upon the depth of fluid in the tank, and the light is reflectively returned from the variable length optical path into the second optical fiber. The two optical fibers are separately pulsed with pulses of light and the light which is returned along the optical fibers is employed to measure the fluid depth. The length measured to the reference point is subtracted from the length measured along the variable path length to yield a measure of the variable portion of the path line alone.